The hope and anticipation felt by Holly Greenhow and her family when their flight touched down in Los Angeles was tempered by not a little trepidation.

It was, they were all acutely aware, a leap into the unknown.

Holly, 11, who captured the nation’s hearts three years ago when she starred in a Boden modelling campaign, has cerebral palsy and has been severely physically disabled since birth. She could neither walk nor talk and it seemed that was how it would always be.

Then, last year, came a flicker of hope. Having been told of a clinic in California offering pioneering stem cell treatment that had produced extraordinary results with stroke victims and cases similar to Holly’s, the Greenhow family boarded a plane in October.

There was a possibility, they were told, that some of the damage to Holly’s brain could be repaired, perhaps even enabling her to walk and talk one day.

It sounded miraculous but the family had no idea what the outcome would be.

‘We knew we had to give it a go,’ says Holly’s mum Fiona. ‘The worst that could happen, we were told, was nothing. It couldn’t harm Holly.’

The family went to see Dr Bryn Henderson, who runs the Regenerative Medical Group, the day after arriving in America, thinking it would be an introductory meeting. But after an ebullient greeting, Dr Henderson said briskly: ‘Let’s get on with it. Today’s gonna change your life.’

Holly was first sedated, then given, via a drip, an infusion of stem cells from the amniotic fluid of women who gave birth by elective Caesarean.

The idea, Dr Henderson told Holly’s parents, 45-year-old Fiona and Paul, 48, was based on a simple premise: the stem cells travel to the area of the body that needs help — in Holly’s case, the damaged part of her brain — then multiply, creating new cells to replace dead ones.